For persons having disabilities which make walking difficult or impossible, a wheelchair has long provided an effective means of transportation. While the wheelchair and occupant may be assisted by a third party, it is more desirable to provide means whereby the occupant may propel himself independently.
The art of providing such means for independent transportation dates back at least before the turn of the century. See for example U.S. Pat. No. 287,789 granted to Arbogast on Nov. 6, 1883, in which the ground wheels of a wheelchair are driven by a chain which is also connected to a hand crank. The chain provides a smooth linkage between the hand crank and the ground wheel; however, to attain the leverage desirable for easy operation, the hand crank must extend to an awkward length.
Later art shows that the desired leverage is achieved if a drive lever, reciprocated by the pumping motion of the occupant's hand and arm, is adapted to provide rotational force to a ground wheel. U.S. Pat. No. 654,986 to Krueger, granted July 31, 1900, and U.S. Pat. No. 838,228 to Williams, granted Dec. 11, 1906, are examples of such a transfer from oscillatory motion of a drive lever to rotational motion of a ground wheel. In these devices, forward motion of the drive lever causes it to engage with drive gears which propel the ground wheel forward. Two problems inherent in the devices are: (1) that for each forward stroke with produces motion there is a rearward return stroke with produces no motion in the wheel, i.e. a "wasted return stroke"; and (2) that the required meshing action of gear teeth produces friction and is a drain on efficiency.
An attempt to produce rotary motion with the return stroke as well as the forward stroke of the drive lever is seen in U.S. Pat. No. 2,547,600 granted to Saxer Apr. 3, 1951, concerning a device designed for propelling a bicycle wheel using the handle bar. A chain connects the bicycle wheel sprocket to a driving sprocket proximate to the handle bar, and reciprocation of the handle bars results in forward movement of the front wheel. Operation of the driving unit is nonetheless still dependant upon the meshing action of gear teeth.
Much simpler systems eliminate the need for the meshing action of gear teeth. See for example U.S. Pat. No. 2,130,426 granted to Henderson Sept. 20, 1938, and U.S. Pat. No. 3,301,574 to Good granted Jan. 31, 1967, both of which involve frictional contact between a drive lever and the perimeter of a ground wheel when the drive lever is pushed in a forward direction. Of course, only the forward stroke produces motion in the wheel and the return stroke is useless.
Cam and follower arrangements are an attempt to eliminate both problems mentioned in connection with Krueger and Williams. Such arrangements are shown in U.S. Pat. No. 1,876,700 granted to H. Lee on Sept. 13, 1932, which shows a mobile kitchen chair; U.S. Pat. No. 3,053,550 to Kunsch et al. on Sept. 11, 1962; and U.S. Pat. No. 3,666,292 granted to Bartos on May 30, 1972. In these systems a follower linked to the drive lever drives a cam linked to the ground wheel axle. There are a number of inherent weaknesses to these systems due to the geometry of the cam and follower arrangement. In the rotation of every cam are two points at which the line of force as applied through the follower passes directly through the center of rotation of the cam. At these points, there can be no tangential force applied to the cam, regardless of the amount of lineal force applied to the follower. This problem is recognized in the Kunsch patent, wherein these points are referred to as "dead centers". Another weakness of the cam and follower system is that the length of the drive lever stroke is defined by the geometry of the arrangement. Consequently, a return stroke of the drive lever cannot be initiated until the forward stroke of the drive lever is completed; otherwise, the ground wheel would be forced to rotate in a reverse direction. Therefore, the size of the required drive stroke cannot be adjusted to accommodate different wheelchair occupants.
U.S. Pat. No. 2,946,602 granted to R. Lee on July 26, 1960, shows a hand-operated wheel linked to the ground wheel by means of a chain. The Lee system is similar to that shown in Arbogast in that it lacks the leverage gained by systems which translate oscillatory motion to rotational motion. U.S. Pat. No. 3,563,568 to Sasse granted Feb. 16, 1971, shows a wheelchair drive which is simply a hand rim provided along the perimeter of the ground wheel, and is an example of the use of a variable ratio gear system in a wheelchair drive.